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<meta name="description" content="C++ 函数   函数是一组一起执行一个任务的语句。每个 C++ 程序都至少有一个函数，即主函数 main() ，所有简单的程序都可以定义其他额外的函数。 您可以把代码划分到不同的函数中。如何划分代码到不同的函数中是由您来决定的，但在逻辑上，划分通常是根据每个函数执行一个特定的任务来进行的。 函数声明告诉编译器函数的名称、返回类型和参数。函数定义提供了函数的实际主体。 C++ 标准库提供了大量的程序可以调用的内置函数。例如，函数 str..">
		
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<h1>C++ <span class="color_h1">函数</span></h1>

<div class="tutintro">
<p>函数是一组一起执行一个任务的语句。每个 C++ 程序都至少有一个函数，即主函数 <b>main()</b> ，所有简单的程序都可以定义其他额外的函数。</p>
<p>您可以把代码划分到不同的函数中。如何划分代码到不同的函数中是由您来决定的，但在逻辑上，划分通常是根据每个函数执行一个特定的任务来进行的。</p>
<p>函数<b>声明</b>告诉编译器函数的名称、返回类型和参数。函数<b>定义</b>提供了函数的实际主体。</p>
<p>C++ 标准库提供了大量的程序可以调用的内置函数。例如，函数 <b>strcat()</b> 用来连接两个字符串，函数 <b>memcpy()</b> 用来复制内存到另一个位置。</p>
<p>函数还有很多叫法，比如方法、子例程或程序，等等。</p>
</div>

<h2 class="tutheader">定义函数</h2>
<p>C++ 中的函数定义的一般形式如下：</p>
<div class="example"> 
<div class="example_code">
<div class="hl-main"><span class="hl-identifier">return_type</span><span class="hl-code"> </span><span class="hl-identifier">function_name</span><span class="hl-brackets">(</span><span class="hl-code"> </span><span class="hl-identifier">parameter</span><span class="hl-code"> </span><span class="hl-identifier">list</span><span class="hl-code"> </span><span class="hl-brackets">)</span><span class="hl-code">
</span><span class="hl-brackets">{</span><span class="hl-code">
   </span><span class="hl-identifier">body</span><span class="hl-code"> </span><span class="hl-identifier">of</span><span class="hl-code"> </span><span class="hl-identifier">the</span><span class="hl-code"> </span><span class="hl-identifier">function</span><span class="hl-code">
</span><span class="hl-brackets">}</span></div>
</div>
</div>
<p>在 C++ 中，函数由一个函数头和一个函数主体组成。下面列出一个函数的所有组成部分：</p>
<ul class="list">
<li><b>返回类型：</b>一个函数可以返回一个值。<b>return_type</b> 是函数返回的值的数据类型。有些函数执行所需的操作而不返回值，在这种情况下，return_type 是关键字 <b>void</b>。</li>
<li><b>函数名称：</b>这是函数的实际名称。函数名和参数列表一起构成了函数签名。</li>
<li><b>参数：</b>参数就像是占位符。当函数被调用时，您向参数传递一个值，这个值被称为实际参数。参数列表包括函数参数的类型、顺序、数量。参数是可选的，也就是说，函数可能不包含参数。</li>
<li><b>函数主体：</b>函数主体包含一组定义函数执行任务的语句。</li>
</ul>

<h2 class="tutheader">实例</h2>
<p>以下是 <b>max()</b> 函数的源代码。该函数有两个参数 num1 和 num2，会返回这两个数中较大的那个数：</p>
<div class="example"> 
<div class="example_code">
<div class="hl-main"><span class="hl-comment">// 函数返回两个数中较大的那个数</span><span class="hl-comment"></span><span class="hl-code">
 
</span><span class="hl-types">int</span><span class="hl-code"> </span><span class="hl-identifier">max</span><span class="hl-brackets">(</span><span class="hl-types">int</span><span class="hl-code"> </span><span class="hl-identifier">num1</span><span class="hl-code">, </span><span class="hl-types">int</span><span class="hl-code"> </span><span class="hl-identifier">num2</span><span class="hl-brackets">)</span><span class="hl-code"> 
</span><span class="hl-brackets">{</span><span class="hl-code">
   </span><span class="hl-comment">// 局部变量声明</span><span class="hl-comment"></span><span class="hl-code">
   </span><span class="hl-types">int</span><span class="hl-code"> </span><span class="hl-identifier">result</span><span class="hl-code">;
 
   </span><span class="hl-reserved">if</span><span class="hl-code"> </span><span class="hl-brackets">(</span><span class="hl-identifier">num1</span><span class="hl-code"> &gt; </span><span class="hl-identifier">num2</span><span class="hl-brackets">)</span><span class="hl-code">
      </span><span class="hl-identifier">result</span><span class="hl-code"> = </span><span class="hl-identifier">num1</span><span class="hl-code">;
   </span><span class="hl-reserved">else</span><span class="hl-code">
      </span><span class="hl-identifier">result</span><span class="hl-code"> = </span><span class="hl-identifier">num2</span><span class="hl-code">;
 
   </span><span class="hl-reserved">return</span><span class="hl-code"> </span><span class="hl-identifier">result</span><span class="hl-code">; 
</span><span class="hl-brackets">}</span></div>
</div>
</div>

<h2 class="tutheader">函数声明</h2>
<p>函数<b>声明</b>会告诉编译器函数名称及如何调用函数。函数的实际主体可以单独定义。</p>
<p>函数声明包括以下几个部分：</p>
<pre>
return_type function_name( parameter list );
</pre>
<p>针对上面定义的函数 max()，以下是函数声明：</p>
<pre>
int max(int num1, int num2);
</pre>
<p>在函数声明中，参数的名称并不重要，只有参数的类型是必需的，因此下面也是有效的声明：</p>
<pre>
int max(int, int);
</pre>
<p>当您在一个源文件中定义函数且在另一个文件中调用函数时，函数声明是必需的。在这种情况下，您应该在调用函数的文件顶部声明函数。</p>

<h2 class="tutheader">调用函数</h2>
<p>创建 C++ 函数时，会定义函数做什么，然后通过调用函数来完成已定义的任务。</p>
<p>当程序调用函数时，程序控制权会转移给被调用的函数。被调用的函数执行已定义的任务，当函数的返回语句被执行时，或到达函数的结束括号时，会把程序控制权交还给主程序。</p>
<p>调用函数时，传递所需参数，如果函数返回一个值，则可以存储返回值。例如：</p>
<div class="example"> 
<h2 class="example">实例</h2>
<div class="example_code">
<div class="hl-main"><span class="hl-prepro">#include</span><span class="hl-prepro"> </span><span class="hl-quotes">&lt;</span><span class="hl-string">iostream</span><span class="hl-quotes">&gt;</span><span class="hl-prepro"></span><span class="hl-code">
</span><span class="hl-reserved">using</span><span class="hl-code"> </span><span class="hl-types">namespace</span><span class="hl-code"> </span><span class="hl-identifier">std</span><span class="hl-code">;
 
</span><span class="hl-comment">// 函数声明</span><span class="hl-comment"></span><span class="hl-code">
</span><span class="hl-types">int</span><span class="hl-code"> </span><span class="hl-identifier">max</span><span class="hl-brackets">(</span><span class="hl-types">int</span><span class="hl-code"> </span><span class="hl-identifier">num1</span><span class="hl-code">, </span><span class="hl-types">int</span><span class="hl-code"> </span><span class="hl-identifier">num2</span><span class="hl-brackets">)</span><span class="hl-code">;
 
</span><span class="hl-types">int</span><span class="hl-code"> </span><span class="hl-identifier">main</span><span class="hl-code"> </span><span class="hl-brackets">(</span><span class="hl-brackets">)</span><span class="hl-code">
</span><span class="hl-brackets">{</span><span class="hl-code">
   </span><span class="hl-comment">// 局部变量声明</span><span class="hl-comment"></span><span class="hl-code">
   </span><span class="hl-types">int</span><span class="hl-code"> </span><span class="hl-identifier">a</span><span class="hl-code"> = </span><span class="hl-number">100</span><span class="hl-code">;
   </span><span class="hl-types">int</span><span class="hl-code"> </span><span class="hl-identifier">b</span><span class="hl-code"> = </span><span class="hl-number">200</span><span class="hl-code">;
   </span><span class="hl-types">int</span><span class="hl-code"> </span><span class="hl-identifier">ret</span><span class="hl-code">;
 
   </span><span class="hl-comment">// 调用函数来获取最大值</span><span class="hl-comment"></span><span class="hl-code">
   </span><span class="hl-identifier">ret</span><span class="hl-code"> = </span><span class="hl-identifier">max</span><span class="hl-brackets">(</span><span class="hl-identifier">a</span><span class="hl-code">, </span><span class="hl-identifier">b</span><span class="hl-brackets">)</span><span class="hl-code">;
 
   </span><span class="hl-identifier">cout</span><span class="hl-code"> &lt;&lt; </span><span class="hl-quotes">&quot;</span><span class="hl-string">Max value is : </span><span class="hl-quotes">&quot;</span><span class="hl-code"> &lt;&lt; </span><span class="hl-identifier">ret</span><span class="hl-code"> &lt;&lt; </span><span class="hl-identifier">endl</span><span class="hl-code">;
 
   </span><span class="hl-reserved">return</span><span class="hl-code"> </span><span class="hl-number">0</span><span class="hl-code">;
</span><span class="hl-brackets">}</span><span class="hl-code">
 
</span><span class="hl-comment">// 函数返回两个数中较大的那个数</span><span class="hl-comment"></span><span class="hl-code">
</span><span class="hl-types">int</span><span class="hl-code"> </span><span class="hl-identifier">max</span><span class="hl-brackets">(</span><span class="hl-types">int</span><span class="hl-code"> </span><span class="hl-identifier">num1</span><span class="hl-code">, </span><span class="hl-types">int</span><span class="hl-code"> </span><span class="hl-identifier">num2</span><span class="hl-brackets">)</span><span class="hl-code"> 
</span><span class="hl-brackets">{</span><span class="hl-code">
   </span><span class="hl-comment">// 局部变量声明</span><span class="hl-comment"></span><span class="hl-code">
   </span><span class="hl-types">int</span><span class="hl-code"> </span><span class="hl-identifier">result</span><span class="hl-code">;
 
   </span><span class="hl-reserved">if</span><span class="hl-code"> </span><span class="hl-brackets">(</span><span class="hl-identifier">num1</span><span class="hl-code"> &gt; </span><span class="hl-identifier">num2</span><span class="hl-brackets">)</span><span class="hl-code">
      </span><span class="hl-identifier">result</span><span class="hl-code"> = </span><span class="hl-identifier">num1</span><span class="hl-code">;
   </span><span class="hl-reserved">else</span><span class="hl-code">
      </span><span class="hl-identifier">result</span><span class="hl-code"> = </span><span class="hl-identifier">num2</span><span class="hl-code">;
 
   </span><span class="hl-reserved">return</span><span class="hl-code"> </span><span class="hl-identifier">result</span><span class="hl-code">; 
</span><span class="hl-brackets">}</span></div>
</div>
</div>
<p>把 max() 函数和 main() 函数放一块，编译源代码。当运行最后的可执行文件时，会产生下列结果：</p>
<pre>
Max value is : 200
</pre>

<h2 class="tutheader">函数参数</h2>
<p>如果函数要使用参数，则必须声明接受参数值的变量。这些变量称为函数的<b>形式参数</b>。</p>
<p>形式参数就像函数内的其他局部变量，在进入函数时被创建，退出函数时被销毁。</p>
<p>当调用函数时，有三种向函数传递参数的方式：</p>
<table class="reference notranslate">
<tr><th width="30%">调用类型</th><th>描述</th></tr>
<tr><td> <a href="../cplusplus/cpp-function-call-by-value.html" title="C++ 中传值方式调用函数">传值调用</a></td><td>该方法把参数的实际值赋值给函数的形式参数。在这种情况下，修改函数内的形式参数对实际参数没有影响。</td> </tr>
<tr><td> <a href="../cplusplus/cpp-function-call-by-pointer.html" title="C++ 中指针方式调用函数">指针调用</a></td><td>该方法把参数的地址赋值给形式参数。在函数内，该地址用于访问调用中要用到的实际参数。这意味着，修改形式参数会影响实际参数。</td> </tr>
<tr><td> <a href="../cplusplus/cpp-function-call-by-reference.html" title="C++ 中引用方式调用函数">引用调用</a></td><td>该方法把参数的引用赋值给形式参数。在函数内，该引用用于访问调用中要用到的实际参数。这意味着，修改形式参数会影响实际参数。</td> </tr>
</table>
<p>默认情况下，C++ 使用<b>传值调用</b>来传递参数。一般来说，这意味着函数内的代码不能改变用于调用函数的参数。之前提到的实例，调用 max() 函数时，使用了相同的方法。</p>

<h2 class="tutheader">参数的默认值</h2>
<p>当您定义一个函数，您可以为参数列表中后边的每一个参数指定默认值。当调用函数时，如果实际参数的值留空，则使用这个默认值。</p>
<p>这是通过在函数定义中使用赋值运算符来为参数赋值的。调用函数时，如果未传递参数的值，则会使用默认值，如果指定了值，则会忽略默认值，使用传递的值。请看下面的实例：</p>
<div class="example"> 
<h2 class="example">实例</h2>
<div class="example_code">
<div class="hl-main"><span class="hl-prepro">#include</span><span class="hl-prepro"> </span><span class="hl-quotes">&lt;</span><span class="hl-string">iostream</span><span class="hl-quotes">&gt;</span><span class="hl-prepro"></span><span class="hl-code">
</span><span class="hl-reserved">using</span><span class="hl-code"> </span><span class="hl-types">namespace</span><span class="hl-code"> </span><span class="hl-identifier">std</span><span class="hl-code">;
 
</span><span class="hl-types">int</span><span class="hl-code"> </span><span class="hl-identifier">sum</span><span class="hl-brackets">(</span><span class="hl-types">int</span><span class="hl-code"> </span><span class="hl-identifier">a</span><span class="hl-code">, </span><span class="hl-types">int</span><span class="hl-code"> </span><span class="hl-identifier">b</span><span class="hl-code">=</span><span class="hl-number">20</span><span class="hl-brackets">)</span><span class="hl-code">
</span><span class="hl-brackets">{</span><span class="hl-code">
  </span><span class="hl-types">int</span><span class="hl-code"> </span><span class="hl-identifier">result</span><span class="hl-code">;
 
  </span><span class="hl-identifier">result</span><span class="hl-code"> = </span><span class="hl-identifier">a</span><span class="hl-code"> + </span><span class="hl-identifier">b</span><span class="hl-code">;
  
  </span><span class="hl-reserved">return</span><span class="hl-code"> </span><span class="hl-brackets">(</span><span class="hl-identifier">result</span><span class="hl-brackets">)</span><span class="hl-code">;
</span><span class="hl-brackets">}</span><span class="hl-code">
 
</span><span class="hl-types">int</span><span class="hl-code"> </span><span class="hl-identifier">main</span><span class="hl-code"> </span><span class="hl-brackets">(</span><span class="hl-brackets">)</span><span class="hl-code">
</span><span class="hl-brackets">{</span><span class="hl-code">
   </span><span class="hl-comment">// 局部变量声明</span><span class="hl-comment"></span><span class="hl-code">
   </span><span class="hl-types">int</span><span class="hl-code"> </span><span class="hl-identifier">a</span><span class="hl-code"> = </span><span class="hl-number">100</span><span class="hl-code">;
   </span><span class="hl-types">int</span><span class="hl-code"> </span><span class="hl-identifier">b</span><span class="hl-code"> = </span><span class="hl-number">200</span><span class="hl-code">;
   </span><span class="hl-types">int</span><span class="hl-code"> </span><span class="hl-identifier">result</span><span class="hl-code">;
 
   </span><span class="hl-comment">// 调用函数来添加值</span><span class="hl-comment"></span><span class="hl-code">
   </span><span class="hl-identifier">result</span><span class="hl-code"> = </span><span class="hl-identifier">sum</span><span class="hl-brackets">(</span><span class="hl-identifier">a</span><span class="hl-code">, </span><span class="hl-identifier">b</span><span class="hl-brackets">)</span><span class="hl-code">;
   </span><span class="hl-identifier">cout</span><span class="hl-code"> &lt;&lt; </span><span class="hl-quotes">&quot;</span><span class="hl-string">Total value is :</span><span class="hl-quotes">&quot;</span><span class="hl-code"> &lt;&lt; </span><span class="hl-identifier">result</span><span class="hl-code"> &lt;&lt; </span><span class="hl-identifier">endl</span><span class="hl-code">;
 
   </span><span class="hl-comment">// 再次调用函数</span><span class="hl-comment"></span><span class="hl-code">
   </span><span class="hl-identifier">result</span><span class="hl-code"> = </span><span class="hl-identifier">sum</span><span class="hl-brackets">(</span><span class="hl-identifier">a</span><span class="hl-brackets">)</span><span class="hl-code">;
   </span><span class="hl-identifier">cout</span><span class="hl-code"> &lt;&lt; </span><span class="hl-quotes">&quot;</span><span class="hl-string">Total value is :</span><span class="hl-quotes">&quot;</span><span class="hl-code"> &lt;&lt; </span><span class="hl-identifier">result</span><span class="hl-code"> &lt;&lt; </span><span class="hl-identifier">endl</span><span class="hl-code">;
 
   </span><span class="hl-reserved">return</span><span class="hl-code"> </span><span class="hl-number">0</span><span class="hl-code">;
</span><span class="hl-brackets">}</span></div>
</div>
</div>
<p>当上面的代码被编译和执行时，它会产生下列结果：</p>
<pre>
Total value is :300
Total value is :120
</pre>
<hr>
<h2>Lambda 函数与表达式</h2>
<p>C++11 提供了对匿名函数的支持,称为 Lambda 函数(也叫 Lambda 表达式)。 </p>
<p>Lambda 表达式把函数看作对象。Lambda 表达式可以像对象一样使用，比如可以将它们赋给变量和作为参数传递，还可以像函数一样对其求值。</p>
<p>Lambda 表达式本质上与函数声明非常类似。Lambda 表达式具体形式如下:</p>
<pre>
[capture](parameters)-&gt;return-type{body}
</pre>
<p>
例如：</p>
<pre>[](int x, int y){ return x &lt; y ; }</pre>
<p>如果没有返回值可以表示为：</p>
<pre>
[capture](parameters){body}
</pre>
<p>
例如：</p>
<pre>[]{ ++global_x; } </pre>
<p>在一个更为复杂的例子中，返回类型可以被明确的指定如下：</p>
<pre>
[](int x, int y) -&gt; int { int z = x + y; return z + x; }
</pre>
<p>本例中，一个临时的参数 z 被创建用来存储中间结果。如同一般的函数，z 的值不会保留到下一次该不具名函数再次被调用时。</p><p>
如果 lambda 函数没有传回值（例如 void），其返回类型可被完全忽略。</p><p>

在Lambda表达式内可以访问当前作用域的变量，这是Lambda表达式的闭包（Closure）行为。 与JavaScript闭包不同，C++变量传递有传值和传引用的区别。可以通过前面的[]来指定：</p>
<pre>
[]      // 沒有定义任何变量。使用未定义变量会引发错误。
[x, &amp;y] // x以传值方式传入（默认），y以引用方式传入。
[&amp;]     // 任何被使用到的外部变量都隐式地以引用方式加以引用。
[=]     // 任何被使用到的外部变量都隐式地以传值方式加以引用。
[&amp;, x]  // x显式地以传值方式加以引用。其余变量以引用方式加以引用。
[=, &amp;z] // z显式地以引用方式加以引用。其余变量以传值方式加以引用。
</pre>
<p>另外有一点需要注意。对于[=]或[&]的形式，lambda 表达式可以直接使用 this 指针。但是，对于[]的形式，如果要使用 this 指针，必须显式传入：</p>
<pre>
[this]() { this-&gt;someFunc(); }();
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		<li class="comment even thread-even depth-1" id="comment-15301"><span class="comt-f">#0</span><div class="comt-avatar wrapper"><i style="font-size:36px;" class="fa fa-user-circle" aria-hidden="true"></i><div class="tooltip"><p><i class="fa fa-user" aria-hidden="true"></i>&nbsp;&nbsp;&nbsp;lqd052</p><p><i class="fa fa-envelope" aria-hidden="true"></i>&nbsp;&nbsp;172***2008@qq.com</p></div><div id="runoobvote-id-15301" data-commid = "15301" class="upvotejs"><a class="upvote"></a> <span class="count">226</span></div></div><div class="comt-main" id="div-comment-15301"><p><strong>Lambda 函数与表达式</strong></p>
<p>Lambda函数的语法定义如下： </p>
<pre>[capture](parameters) mutable -&gt;return-type{statement}</pre>
<p><strong>其中：</strong></p>
<ul><li>
<strong>[capture]</strong>：捕捉列表。捕捉列表总是出现在 lambda 表达式的开始处。事实上，[] 是 lambda 引出符。编译器根据该引出符判断接下来的代码是否是 lambda 函数。捕捉列表能够捕捉上下文中的变量供 lambda 函数使用。</li><li>
(parameters)：参数列表。与普通函数的参数列表一致。如果不需要参数传递，则可以连同括号 () 一起省略。</li><li>
<strong>mutable</strong>：mutable 修饰符。默认情况下，lambda 函数总是一个 const 函数，mutable 可以取消其常量性。在使用该修饰符时，参数列表不可省略（即使参数为空）。</li><li>
<strong>->return_type</strong>：返回类型。用追踪返回类型形式声明函数的返回类型。出于方便，不需要返回值的时候也可以连同符号 -> 一起省略。此外，在返回类型明确的情况下，也可以省略该部分，让编译器对返回类型进行推导。</li><li>
<strong>{statement}</strong>：函数体。内容与普通函数一样，不过除了可以使用参数之外，还可以使用所有捕获的变量。</li></ul>
<p>
在 lambda 函数的定义式中，参数列表和返回类型都是可选部分，而捕捉列表和函数体都可能为空，C++ 中最简单的 lambda 函数只需要声明为：
</p><pre>
[]{};</pre>
<div class="comt-meta wrapper"><span class="comt-author"><a target="_blank" href="javascript:;">lqd052</a><div class="tooltip tooltip1"><p><i class="fa fa-user" aria-hidden="true"></i>&nbsp;&nbsp;&nbsp;lqd052</p><p><i class="fa fa-envelope" aria-hidden="true"></i>&nbsp;&nbsp;172***2008@qq.com</p></div></span>6年前 (2017-07-24)</div></div></li><!-- #comment-## -->
<li class="comment odd alt thread-odd thread-alt depth-1" id="comment-17245"><span class="comt-f">#0</span><div class="comt-avatar wrapper"><i style="font-size:36px;" class="fa fa-user-circle" aria-hidden="true"></i><div class="tooltip"><p><i class="fa fa-user" aria-hidden="true"></i>&nbsp;&nbsp;&nbsp;Tmacww</p><p><i class="fa fa-envelope" aria-hidden="true"></i>&nbsp;&nbsp;825***602@qq.com</p><p><i class="fa fa-external-link" aria-hidden="true"></i> <a rel="nofollow" target="_blank" href="http://blog.csdn.net/fjzpdkf/article/details/50249287">&nbsp;&nbsp;参考地址</a></p></div><div id="runoobvote-id-17245" data-commid = "17245" class="upvotejs"><a class="upvote"></a> <span class="count">52</span></div></div><div class="comt-main" id="div-comment-17245"><p>两个实际应用到lambda表达式的代码。</p>
<pre>
std::vector&lt;int&gt; v = { 1, 2, 3, 4, 5, 6 };
int even_count = 0;
for_each(v.begin(), v.end(), [&amp;even_count](int val){
    if(!(val &amp; 1)){
        ++ even_count;
    }
});
std::cout &lt;&lt; "The number of even is " &lt;&lt; even_count &lt;&lt; std::endl;
</pre>
<pre>
int count = std::count_if( coll.begin(), coll.end(), [](int x){ return x &gt; 10; });  
  
int count = std::count_if( coll.begin(), coll.end(), [](int x){ return x &lt; 10; });  
  
int count = std::count_if( coll.begin(), coll.end(), [](int x){ return x &gt; 5 &amp;&amp; x&lt;10; });  
</pre><div class="comt-meta wrapper"><span class="comt-author"><a target="_blank" href="javascript:;">Tmacww</a><div class="tooltip tooltip1"><p><i class="fa fa-user" aria-hidden="true"></i>&nbsp;&nbsp;&nbsp;Tmacww</p><p><i class="fa fa-envelope" aria-hidden="true"></i>&nbsp;&nbsp;825***602@qq.com</p><p><i class="fa fa-external-link" aria-hidden="true"></i> <a rel="nofollow" target="_blank" href="http://blog.csdn.net/fjzpdkf/article/details/50249287">&nbsp;&nbsp;参考地址</a></p></div></span>6年前 (2017-09-07)</div></div></li><!-- #comment-## -->
<li class="comment even thread-even depth-1" id="comment-18871"><span class="comt-f">#0</span><div class="comt-avatar wrapper"><i style="font-size:36px;" class="fa fa-user-circle" aria-hidden="true"></i><div class="tooltip"><p><i class="fa fa-user" aria-hidden="true"></i>&nbsp;&nbsp;&nbsp;prigioni</p><p><i class="fa fa-envelope" aria-hidden="true"></i>&nbsp;&nbsp;763***799@qq.com</p></div><div id="runoobvote-id-18871" data-commid = "18871" class="upvotejs"><a class="upvote"></a> <span class="count">61</span></div></div><div class="comt-main" id="div-comment-18871"><p>定义一个可以输出字符串的lambda表达式，表达式一般都是从方括号[]开始，然后结束于花括号{}，花括号里面就像定义函数那样，包含了lamdba表达式体：</p>

<pre>// 定义简单的lambda表达式
auto basicLambda = [] { cout &lt;&lt; "Hello, world!" &lt;&lt; endl; };

// 调用
basicLambda();
// 输出：Hello, world!</pre>

<p>如果需要参数，那么就要像函数那样，放在圆括号里面，如果有返回值，返回类型要放在->后面，即拖尾返回类型，当然你也可以忽略返回类型，lambda会帮你自动推断出返回类型：</p>

<pre>// 指明返回类型
auto add = [](int a, int b) -&gt; int { return a + b; };

// 自动推断返回类型
auto multiply = [](int a, int b) { return a * b; };
int sum = add(2, 5);   // 输出：7
int product = multiply(2, 5);  // 输出：10</pre><div class="comt-meta wrapper"><span class="comt-author"><a target="_blank" href="javascript:;">prigioni</a><div class="tooltip tooltip1"><p><i class="fa fa-user" aria-hidden="true"></i>&nbsp;&nbsp;&nbsp;prigioni</p><p><i class="fa fa-envelope" aria-hidden="true"></i>&nbsp;&nbsp;763***799@qq.com</p></div></span>6年前 (2017-10-15)</div></div></li><!-- #comment-## -->
<li class="comment odd alt thread-odd thread-alt depth-1" id="comment-22197"><span class="comt-f">#0</span><div class="comt-avatar wrapper"><i style="font-size:36px;" class="fa fa-user-circle" aria-hidden="true"></i><div class="tooltip"><p><i class="fa fa-user" aria-hidden="true"></i>&nbsp;&nbsp;&nbsp;eric</p><p><i class="fa fa-envelope" aria-hidden="true"></i>&nbsp;&nbsp;gxy***@163.com</p></div><div id="runoobvote-id-22197" data-commid = "22197" class="upvotejs"><a class="upvote"></a> <span class="count">96</span></div></div><div class="comt-main" id="div-comment-22197"><p>Lambda函数很简洁，但变化较多。</p>

<p><strong>1、什么也不捕获，或者是故意不用 Lambda 函数外部的变量</strong></p>

<pre>#include &lt;iostream&gt;
using namespace std;
 
int main()
{
    int i = 1024;
    auto func = [] { cout &lt;&lt; i; };
    func();
}  // 结果报错，因为未指定默认捕获模式</pre>
<p>正确的如下：</p>

<pre>#include &lt;iostream&gt;
using namespace std;
 
int main()
{
    int i = 1024;
    auto func = [=]{  // [=] 表明将外部的所有变量拷贝一份到该Lambda函数内部
        cout &lt;&lt; i;
    };
    func();
}</pre>
<p><strong>2、引用捕获</strong></p>
<pre>#include &lt;iostream&gt;
using namespace std;
 
int main()
{
    int i = 1024;
    cout &lt;&lt; &amp;i &lt;&lt; endl;
    auto fun1 = [&amp;]{
        cout &lt;&lt; &amp;i &lt;&lt; endl;
    };
    fun1();
}</pre>
<p><strong>3、复制并引用捕获</strong></p>
<pre>#include &lt;iostream&gt;
using namespace std;
 
int main()
{
    int i = 1024, j = 2048;
 
    cout &lt;&lt; "i:" &lt;&lt; &amp;i &lt;&lt; endl;
    cout &lt;&lt; "j:" &lt;&lt; &amp;j &lt;&lt; endl;
 
    auto fun1 = [=, &amp;i]{ // 默认拷贝外部所有变量，但引用变量 i
        cout &lt;&lt; "i:" &lt;&lt; &amp;i &lt;&lt; endl;
        cout &lt;&lt; "j:" &lt;&lt; &amp;j &lt;&lt; endl;
    };
    fun1();
}</pre>
<p><strong>4、指定引用或复制</strong></p>
<pre>#include &lt;iostream&gt;
using namespace std;
 
int main()
{
    int i = 1024, j = 2048;
 
    cout &lt;&lt; "outside i value:" &lt;&lt; i &lt;&lt; " addr:" &lt;&lt; &amp;i &lt;&lt; endl;
 
    auto fun1 = [i]{
        cout &lt;&lt; "inside  i value:" &lt;&lt; i &lt;&lt; " addr:" &lt;&lt; &amp;i &lt;&lt; endl;
        // cout &lt;&lt; j &lt;&lt; endl; // j 未捕获
    };
    fun1();
}</pre>
<p><strong>5、捕获this指针</strong></p>
<pre>#include &lt;iostream&gt;
using namespace std;
 
class test
{
public:
    void hello() {
        cout &lt;&lt; "test hello!n";
    };
    void lambda() {
        auto fun = [this]{ // 捕获了 this 指针
            this-&gt;hello(); // 这里 this 调用的就是 class test 的对象了
        };
        fun();
    }
};
 
int main()
{
    test t;
    t.lambda();
}</pre>
<div class="comt-meta wrapper"><span class="comt-author"><a target="_blank" href="javascript:;">eric</a><div class="tooltip tooltip1"><p><i class="fa fa-user" aria-hidden="true"></i>&nbsp;&nbsp;&nbsp;eric</p><p><i class="fa fa-envelope" aria-hidden="true"></i>&nbsp;&nbsp;gxy***@163.com</p></div></span>5年前 (2017-12-27)</div></div></li><!-- #comment-## -->
<li class="comment even thread-even depth-1" id="comment-25720"><span class="comt-f">#0</span><div class="comt-avatar wrapper"><i style="font-size:36px;" class="fa fa-user-circle" aria-hidden="true"></i><div class="tooltip"><p><i class="fa fa-user" aria-hidden="true"></i>&nbsp;&nbsp;&nbsp;突然的幸福</p><p><i class="fa fa-envelope" aria-hidden="true"></i>&nbsp;&nbsp;292***0456@qq.com</p></div><div id="runoobvote-id-25720" data-commid = "25720" class="upvotejs"><a class="upvote"></a> <span class="count">73</span></div></div><div class="comt-main" id="div-comment-25720"><ul><li>[]：默认不捕获任何变量；</li><li>[=]：默认以值捕获所有变量；</li><li>[&amp;]：默认以引用捕获所有变量；</li><li>[x]：仅以值捕获x，其它变量不捕获；</li><li>[&amp;x]：仅以引用捕获x，其它变量不捕获；</li><li>[=, &amp;x]：默认以值捕获所有变量，但是x是例外，通过引用捕获；</li><li>[&amp;, x]：默认以引用捕获所有变量，但是x是例外，通过值捕获；</li><li>[this]：通过引用捕获当前对象（其实是复制指针）；</li><li>[*this]：通过传值方式捕获当前对象；</li></ul><div class="comt-meta wrapper"><span class="comt-author"><a target="_blank" href="javascript:;">突然的幸福</a><div class="tooltip tooltip1"><p><i class="fa fa-user" aria-hidden="true"></i>&nbsp;&nbsp;&nbsp;突然的幸福</p><p><i class="fa fa-envelope" aria-hidden="true"></i>&nbsp;&nbsp;292***0456@qq.com</p></div></span>5年前 (2018-03-30)</div></div></li><!-- #comment-## -->
<li class="comment odd alt thread-odd thread-alt depth-1" id="comment-25722"><span class="comt-f">#0</span><div class="comt-avatar wrapper"><i style="font-size:36px;" class="fa fa-user-circle" aria-hidden="true"></i><div class="tooltip"><p><i class="fa fa-user" aria-hidden="true"></i>&nbsp;&nbsp;&nbsp;突然的幸福</p><p><i class="fa fa-envelope" aria-hidden="true"></i>&nbsp;&nbsp;292***0456@qq.com</p></div><div id="runoobvote-id-25722" data-commid = "25722" class="upvotejs"><a class="upvote"></a> <span class="count">28</span></div></div><div class="comt-main" id="div-comment-25722"><p>lambda表达式无法修改通过复制形式捕捉的变量，因为函数调用运算符的重载方法是const属性的。有时候，你想改动传值方式捕获的值，那么就要使用mutable，例子如下：</p>

<pre>int main()
{
    int x = 10;

    auto add_x = [x](int a) mutable { x *= 2; return a + x; };  // 复制捕捉x

    cout &lt;&lt; add_x(10) &lt;&lt; endl; // 输出 30
    return 0;
}</pre><div class="comt-meta wrapper"><span class="comt-author"><a target="_blank" href="javascript:;">突然的幸福</a><div class="tooltip tooltip1"><p><i class="fa fa-user" aria-hidden="true"></i>&nbsp;&nbsp;&nbsp;突然的幸福</p><p><i class="fa fa-envelope" aria-hidden="true"></i>&nbsp;&nbsp;292***0456@qq.com</p></div></span>5年前 (2018-03-30)</div></div></li><!-- #comment-## -->
<li class="comment even thread-even depth-1" id="comment-38802"><span class="comt-f">#0</span><div class="comt-avatar wrapper"><i style="font-size:36px;" class="fa fa-user-circle" aria-hidden="true"></i><div class="tooltip"><p><i class="fa fa-user" aria-hidden="true"></i>&nbsp;&nbsp;&nbsp;hanxiaolong2333</p><p><i class="fa fa-envelope" aria-hidden="true"></i>&nbsp;&nbsp;320***804@qq.com</p></div><div id="runoobvote-id-38802" data-commid = "38802" class="upvotejs"><a class="upvote"></a> <span class="count">23</span></div></div><div class="comt-main" id="div-comment-38802"><p>编程语言中，函数直接或间接调用函数本身，则该函数称为递归函数。</p>

<pre>#include &lt;iostream&gt;

using namespace std;

// 本程序用递归算法求n的阶乘
int counter=1;

int fac(int n)
{
    cout&lt;&lt;"第"&lt;&lt;counter&lt;&lt;"次调用："&lt;&lt;n&lt;&lt;" * "&lt;&lt;"fac("&lt;&lt;n-1&lt;&lt;")"&lt;&lt;endl;
    if (n==1) {
        return 1;
    }
    else
    { 
        counter++;
        return n * fac(n-1); // 断点1
    }
}

int main()
{ 
    int n;
    cout&lt;&lt;"请输入求n的阶乘的n值：";
    cin&gt;&gt;n;
    cout&lt;&lt;"结果：fac("&lt;&lt;n&lt;&lt;")="&lt;&lt;fac(n)&lt;&lt;endl;
}</pre><div class="comt-meta wrapper"><span class="comt-author"><a target="_blank" href="javascript:;">hanxiaolong2333</a><div class="tooltip tooltip1"><p><i class="fa fa-user" aria-hidden="true"></i>&nbsp;&nbsp;&nbsp;hanxiaolong2333</p><p><i class="fa fa-envelope" aria-hidden="true"></i>&nbsp;&nbsp;320***804@qq.com</p></div></span>4年前 (2019-01-09)</div></div></li><!-- #comment-## -->
<li class="comment odd alt thread-odd thread-alt depth-1" id="comment-57167"><span class="comt-f">#0</span><div class="comt-avatar wrapper"><i style="font-size:36px;" class="fa fa-user-circle" aria-hidden="true"></i><div class="tooltip"><p><i class="fa fa-user" aria-hidden="true"></i>&nbsp;&nbsp;&nbsp;Buktop</p><p><i class="fa fa-envelope" aria-hidden="true"></i>&nbsp;&nbsp;931***331@qq.com</p></div><div id="runoobvote-id-57167" data-commid = "57167" class="upvotejs"><a class="upvote"></a> <span class="count">33</span></div></div><div class="comt-main" id="div-comment-57167"><p><strong>一个新手可能会犯的错误</strong></p>
<p>
由于在实际写代码过程中，可能会与到函数的定义和声明不在同一文件，声明出现在头文件中的情况</p>
<p>
这时，对带默认值的函数在声明和定义时，需要注意，默认值出现在声明的代码中，定义中不能再对参数赋与默认值，否则会报错。</p>

<pre>&lt;Plus.h&gt; //声明在头文件中
int Plus(int a, int b = 10)
&lt;Plus.cpp&gt;//定义在cpp文件中
int Plus(int a, int b)//不能再对b赋值了
{
    return a + b;
}</pre>
<div class="comt-meta wrapper"><span class="comt-author"><a target="_blank" href="javascript:;">Buktop</a><div class="tooltip tooltip1"><p><i class="fa fa-user" aria-hidden="true"></i>&nbsp;&nbsp;&nbsp;Buktop</p><p><i class="fa fa-envelope" aria-hidden="true"></i>&nbsp;&nbsp;931***331@qq.com</p></div></span>3年前 (2020-09-19)</div></div></li><!-- #comment-## -->
<li class="comment byuser comment-author-tianqixin even thread-even depth-1" id="comment-59641"><span class="comt-f">#0</span><div class="comt-avatar wrapper"><i style="font-size:36px;" class="fa fa-user-circle" aria-hidden="true"></i><div class="tooltip"><p><i class="fa fa-user" aria-hidden="true"></i>&nbsp;&nbsp;&nbsp;VALUE</p><p><i class="fa fa-envelope" aria-hidden="true"></i>&nbsp;&nbsp;phj***89905610@163.com</p></div><div id="runoobvote-id-59641" data-commid = "59641" class="upvotejs"><a class="upvote"></a> <span class="count">19</span></div></div><div class="comt-main" id="div-comment-59641"><p>函数必须声明才能使用：</p>

<pre>#include &lt;iostream&gt;
#include&lt;limits&gt;
using namespace std;
//float area(float a,float b);
int main()
{
  
    float length,width;
    length=10.0;
    width=5.0;
    float areas=area(length,width);
    cout &lt;&lt; areas &lt;&lt;endl;
    return 0;
}
float area(float a,float b){
    return a*b;
}</pre>
<p>报错：</p>

<pre>main.cpp: In function &lsquo;int main()&rsquo;:
main.cpp:11:14: error: &lsquo;area&rsquo; was not declared in this scope; did you mean &lsquo;areas&rsquo;?
   11 |  float areas=area(length,width);
      |              ^~~~
      |              areas</pre>
<p>正确做法：</p>

<pre>#include &lt;iostream&gt;
#include&lt;limits&gt;
using namespace std;
float area(float a,float b);// 声明函数
int main()
{
  
    float length,width;
    length=10.0;
    width=5.0;
    float areas=area(length,width);// 调用函数
    cout &lt;&lt; areas &lt;&lt;endl;
    return 0;
}
// 定义函数
float area(float a,float b){
    return a*b;
}</pre>
<p>运行结果：</p>

<pre>50</pre><div class="comt-meta wrapper"><span class="comt-author"><a target="_blank" href="javascript:;">VALUE</a><div class="tooltip tooltip1"><p><i class="fa fa-user" aria-hidden="true"></i>&nbsp;&nbsp;&nbsp;VALUE</p><p><i class="fa fa-envelope" aria-hidden="true"></i>&nbsp;&nbsp;phj***89905610@163.com</p></div></span>2年前 (2020-12-18)</div></div></li><!-- #comment-## -->
<li class="comment odd alt thread-odd thread-alt depth-1" id="comment-73840"><span class="comt-f">#0</span><div class="comt-avatar wrapper"><i style="font-size:36px;" class="fa fa-user-circle" aria-hidden="true"></i><div class="tooltip"><p><i class="fa fa-user" aria-hidden="true"></i>&nbsp;&nbsp;&nbsp;2020.7.11</p><p><i class="fa fa-envelope" aria-hidden="true"></i>&nbsp;&nbsp;191***0290@qq.com</p></div><div id="runoobvote-id-73840" data-commid = "73840" class="upvotejs"><a class="upvote"></a> <span class="count">4</span></div></div><div class="comt-main" id="div-comment-73840"><p><strong>特殊说明</strong></p>
<p>
<span class="marked">[this]</span>: 通过引用捕获当前对象（对象本身）</p><p>
<span class="marked">[*this]</span>: 通过传值捕获当前对象（对象拷贝，且只是一个地址值，可以说没有意义）</p>
<p>举例说明：</p>

<pre>#include &lt;iostream&gt;
using namepsace std;

//定义一个类
class MM 
{
public:
  void print() {}
};

int main()
{
  MM mm;

  //打印对象地址作对比
  cout &lt;&lt; "对象本身地址：" &lt;&lt; &amp;mm &lt;&lt; endl;
  
  [&amp;mm](){cout &lt;&lt; "[&amp;mm]: " &lt;&lt; &amp;mm &lt;&lt; endl;}();    //相当于[this]
  [mm](){cout &lt;&lt; "[mm]: " &lt;&lt; &amp;mm &lt;&lt; endl;}();      //相当于[*this]
  [mm](){cout &lt;&lt; "[mm]: " &lt;&lt; &amp;mm &lt;&lt; endl;}();      //相当于[*this]

  return 0;
}</pre>
<p>结果：</p>

<pre>对象本身地址：012FF984
[&amp;mm]: 012FF984
[mm]: 012FF878
[mm]: 012FF850</pre>
<p>尝试将捕获的指针调用成员函数：</p>

<pre>[&amp;mm](){cout &lt;&lt; this-&gt;print();}();    //相当于[this]
//[mm](){cout &lt;&lt; this-&gt;print();}();    //相当于[*this]，无法调用，因为传入的只是一个值</pre><div class="comt-meta wrapper"><span class="comt-author"><a target="_blank" href="javascript:;">2020.7.11</a><div class="tooltip tooltip1"><p><i class="fa fa-user" aria-hidden="true"></i>&nbsp;&nbsp;&nbsp;2020.7.11</p><p><i class="fa fa-envelope" aria-hidden="true"></i>&nbsp;&nbsp;191***0290@qq.com</p></div></span>10个月前 (07-11)</div></div></li><!-- #comment-## -->
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		var _wp_unfiltered_html_comment = $("#_wp_unfiltered_html_comment").val();
		var comment = noteContent;
		var author = $("#author").val();
		var url = $("#url").val();
		var email = $("#email").val();
		if(isBlank(author) && is_user_logged_in==0) {
			statusdiv.html('<p  class="ajax-error">请输入昵称！</p>').show();
		} else if(isBlank(email)  && is_user_logged_in==0) {
			statusdiv.html('<p  class="ajax-error">请输入邮箱！</p>').show();
		} else {
			// var formdata=commentform.serialize() + "&comment=" + noteContent ;
			// 添加状态信息
			statusdiv.html('<p>Processing...</p>').show();
			// 获取表单提交地址
			var formurl=commentform.attr('action');
			
			// 异步提交
			$.ajax({
					type: 'post',
					url: formurl,
					dataType:'json',
					data: {"comment_parent":comment_parent,"comment_post_ID":comment_post_ID, "_wp_unfiltered_html_comment":_wp_unfiltered_html_comment,"comment":comment,"url":url, "email":email,"author":author},
					error: function(XMLHttpRequest, textStatus, errorThrown){
					statusdiv.html('<p class="ajax-error" >数据不完整或表单提交太快了！</p>').show();
				},
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						alert('笔记已提交审核，感谢分享笔记！');
					}else{
						statusdiv.html('<p class="ajax-error" >'+data.msg+'</p>').show();
					}
					commentform.find('textarea[name=comment]').val('');
				}
			});
			setTimeout(function(){
		        $("#submit").prop('disabled', false);
		    }, 10*1000);
		}
		return false;

	});
	$(".comt-author").click(function() {
		href = $(this).children("a").attr("href");
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	});
	*/
	//Upvote.create('runoobvote-id', {callback: vote_callback});
	var ajaxurl = 'https://www.runoob.com/wp-admin/admin-ajax.php';
	var callback = function(data) {
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		//console.log(data);
		_vote_action = data.action;
		id_arr = data.id.split('-');
		um_id= id_arr[2];
		//console.log(um_id);
		
		var re = /^[1-9]+/;
		if (re.test(um_id)) { 
			var ajax_data = {
				_vote_action: _vote_action,
				action: "pinglun_zan",
				um_id: um_id,
				um_action: "ding"
			};
			//console.log(ajax_data);
			$.post(ajaxurl,ajax_data,function(status){
				//if(status.vote_num>999) {
				//	_voteHtml = '<span style="display: block; text-align: center;font-size: 20px; color: #6a737c; margin: 8px 0;">'+kFormatter(status.vote_num) +'</span>';
				//	$("#runoobvote-id-" + um_id + " .count").hide().after(_voteHtml);
				//}
				
			});
		}
	};
	if($('#comments').length && $('.upvotejs').length){
		$('.upvotejs').upvote({id: 11180, callback: callback});
	
		$.post(ajaxurl,{"action":"pinglun_zan","postid":11180},function(data){  
			$(data).each(function(key,value) {
				$("#runoobvote-id-" + value.commid + " .upvote").addClass(value.upvotejs_class);
				$("#runoobvote-id-" + value.commid + " .downvote").addClass(value.downvote_class);
				$("#runoobvote-id-" + value.commid + " .count").text(value.upvote_count);
			})
		},'json');
		
	}
	
	
});
function isBlank(str) {
    return (!str || /^\s*$/.test(str));
}
function kFormatter(num) {
	// return num;
    return Math.abs(num) > 999 ? Math.sign(num)*((Math.abs(num)/1000).toFixed(1)) + 'k' : Math.sign(num)*Math.abs(num)
}

</script>

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